1. Field of the Invention
The present invention relates to a grinding and/or dispersing agent, a method of grinding and/or dispersing with such an agent and a dispersion containing said agent.
2. Discussion of the Background
For a long time, conventional practice has included the use of mineral substances such as calcium carbonates and titanium dioxide for the preparation of industrial products intended for paints, paper coating, fillers for rubbers and synthetic resins, and others.
However, since these mineral substances do not have a natural lamellar or laminated structure facilitating their splitting, as is the case for certain substances such as aluminum silicates, generally known as kaolin, the specialist must, in order to use them in the pigment field, transform them by grinding in a very fine aqueous suspension whose constituent grains are as small as possible, i.e., less than several microns.
Through the publications that have appeared in this area, the specialized literature reveals the importance and complexity of grinding in an aqueous medium of mineral substances, so as to obtain a quality of partial fineness permitting pigmentary applications. Thus, it is well known, in the special case of paper coating, that the coating material composed of mineral pigments such as kaolins, calcium carbonates, and titanium dioxides are placed in suspension in water, also contains binding agents and dispersants, as well as other additives such as thickening agents and coloring agents.
It is desirable to ensure that a suspension of this kind possesses low, stable viscosity for the length of time it is stored, so as to facilitate handling and use, and, similarly, has as high a mineral substance content as possible, in order to reduce the quantity of water handled. A suspension, ideally uniting all of these basic properties, would allow the skilled artisan to solve the well-known problems of grinding, storage, transport from the production site to the application site, and, finally, of transfer by pump during use.
Thus, to date those of skill in the art have been familiar with the use of grinding and/or aqueous dispersing agents for mineral particles formed by acrylic polymers and/or copolymers which are totally or partially neutralized by various neutralization agents (Patents Nos. FR2539137, FR2531444, FR2603042).
The specialist is also familiar with the use of a grinding and/or dispersing agent formed from the fraction of alkali acrylic polymers and/or copolymers totally neutralized by a single cation, and whose specific viscosity is between 0.3 and 0.8 (Patent No. FR2488814), or which is partially neutralized by a single cation such as sodium, potassium, lithium, or ammonium (EP0127388; EP0185458).
These various types of grinding and/or dispersing agents, according to prior art and other treatment or grinding procedures known to the artisan (EP0278880; FR2642415), make it possible to obtain aqueous suspensions of fine mineral particles, which have been thought until now, to be stable over time, but for which the user experiences some handling problems, since they are often stored for several days or weeks without stirring, and thus must deal with an increase in viscosity of the unstirred suspensions, called "the gel effect".
In fact, in prior art, the viscosity of the aqueous suspensions of mineral particles was generally measured using a Brookfield RVT-type viscosimeter at a temperature of 20.degree. C. and a rate of rotation of 100 revolutions/minute:
(a) at the grinding apparatus outlet; PA1 (b) after eight days of rest in the bottle and after preliminary stirring of the resting suspension.
These two measurement parameters did not make it possible to bring to light "the gel effect" of the aqueous mineral suspension, which is troublesome to the user, who must handle concentrated suspensions which have been stored for several days unstirred, and characterized by a Brookfield viscosity before stirring of more than 5,000 mPa.s, obtained by measuring at 10 revolutions/minute.
Faced with these problems, the present inventors have found that the aqueous mineral suspensions obtained according to prior art had high Brookfield viscosities, if the measurement was made after extended storage without stirring the suspension preliminarily, as is generally the case the user of these suspensions is faced with.
Based on this finding, the present inventors have developed a grinding and/or dispersing agent and a grinding procedure in the presence of this agent, which, surprisingly, make it possible to produce aqueous mineral suspensions meeting the aforementioned quality criteria.
Surprisingly, the objectives of the invention are achieved when between 40 and 60%, of the active acid sites of an acrylic polymer including terminals, are neutralized by a neutralization agent containing magnesium ions, while less than, or equal to 60% of the active acid sites of the polymer are neutralized by a neutralizing agent containing monovalent cations.